The output power amplifier of a cellular telephone handset or mobile data device consumes a substantial amount of power when voice or data are being transmitted. In most cellular telephony protocols, the transmitted power at the mobile station is intentionally varied; for example, when a code-division-multiple-access (CDMA) protocol is employed, the transmitted power for each mobile is adjusted to ensure that the received power from each mobile at the basestation is approximately constant. As a result of transmit power control, mobile stations typically transmit at much lower than the maximum power level they can attain. For example, in a conventional CDMA system, the mobile station may be capable of transmitting up to 1 watt (30 dBm) of RF output power, but will most often transmit at a power level of 3 to 10 milliwatts, as depicted in FIG. 1.
In order to ensure good linearity during operation, mobile station power amplifiers operate in class A or class AB mode, where substantial power is consumed due to the bias current irrespective of the signal power transmitted. In this case, it is wasteful to employ a high supply voltage when only a low-power signal needs to be sent. To minimize the DC power required for the AC power to be transmitted, some implementations include intentionally varying the DC supply voltage or the DC current provide to the power amplifier. The supply voltage may be varied to follow the “instantaneous” envelope of the transmitted signal, or from one packet to the next based on the nominal power of the packet. or less frequently based on the current average transmit power requested from the Power Amplifier.
However, the described schemes have several disadvantages. Specialized DC-DC converters are required, along with complex control circuitry to match the instantaneous supplied voltage with the output power required. Even in the case where only slow voltage adaptation is attempted, a dedicated variable-output-voltage DC-DC converter is required for the power amplifier, incurring added cost. It is therefore desirable to have a lower-cost means of improving system efficiency.